Impact of fertilization and thinning on tree resistance to mountain pine beetle (Dendroctonus ponderosae) and associated fungi

The initial step in the resistance sequence of conifers to bark beetle attack is wound cleansing (resin flow) at the point of attack. Silvicultural treatments such as the thinning and/or fertilization of stands can impact this resistance parameter. We examined the impact of stand thinning and fertilization on the resin flow and inner bark monoterpene content of lodgepole pines, one of the primary hosts of the mountain pine beetle. There were measurable differences among treatments for both of these parameters. In addition, fertilized trees had a significantly increased inner bark nitrogen content. This increased inner bark content resulted in a measurable increase in beetle nitrogen content and size following fertilization. The fungi associated with mountain pine beetle may also be impacted by the increased nitrogen content of the inner bark. For example, in a complimentary study we found significantly increased nitrogen content of the media on which it grows also increases the nitrogen content of two of the fungi associated with mountain pine beetle, Ophiostoma montium and O. clavigerum. If the beetle feeds upon the fungi prior to emerging from a host tree, the increased nitrogen content may influence beetle longevity and or reproduction.